Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
394 views
in Technique[技术] by (71.8m points)

sql - Pivot table in MySQL - convert to pivot table with values in varchar format

i would like convert data from Table1 as you can see on first picture on data in Pivot_table. Is possible to do that in MySQL? Because the values of pivot table(A, B, C, D) are in varchar data format and i cannot use any aggregation function of MySQL like SUM or others.

Table1:
PK        Name     Subject     Grade
-------------------------------------
1         Bob       Math        A
2         Bob       History     B
3         Bob       Language    C
4         Bob       Biology     D
5         Sue       History     C
6         Sue       Math        A
7         Sue       Music       A
8         Sue       Geography   C


Pivot_table:
Subject     Bob     Sue
-------------------------
Math        A        A
History     B        C
Language    C 
Biology     D
Music                A
Geography            C

Thanks for your help

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

A static query (as far as only Bob and Sue are concerned) might look like this

SELECT subject, 
       MAX(CASE WHEN name = 'Bob' THEN grade END) `Bob`,
       MAX(CASE WHEN name = 'Sue' THEN grade END) `Sue` 
  FROM table1 
 GROUP BY subject

Now to be able to account for other names use dynamic SQL like this

SET @sql = NULL;

SELECT GROUP_CONCAT(DISTINCT
         CONCAT('MAX(CASE WHEN name = ''', name,
                ''' THEN grade END) `', name, '`'))
  INTO @sql
  FROM table1;

SET @sql = CONCAT('SELECT subject, ', @sql, ' 
                     FROM table1 
                    GROUP BY subject');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Output:

|   SUBJECT |    BOB |    SUE |
|-----------|--------|--------|
|   Biology |      D | (null) |
| Geography | (null) |      C |
|   History |      B |      C |
|  Language |      C | (null) |
|      Math |      A |      A |
|     Music | (null) |      A |

Here is SQLFiddle demo


You can wrap it into a stored procedure to simplify things on the calling end

DELIMITER $$
CREATE PROCEDURE sp_grade_report()
BEGIN
  SET @sql = NULL;

  SELECT GROUP_CONCAT(DISTINCT
           CONCAT('MAX(CASE WHEN name = ''', name,
                  ''' THEN grade END) `', name, '`'))
    INTO @sql
    FROM table1;

  SET @sql = CONCAT('SELECT subject, ', @sql, ' 
                       FROM table1 
                      GROUP BY subject');

  PREPARE stmt FROM @sql;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;

Sample usage:

CALL sp_grade_report();

Here is SQLFiddle demo


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...